Process for dehydration of dilute ethanol into ethylene with low energy consumption without recycling of water

ABSTRACT

A process for dehydration of an ethanol feedstock into ethylene, comprising the vaporization of said dilute hydrated ethanol feedstock in an exchanger, with heat exchange with the effluent that is obtained from a last reactor, with said mixture being introduced into said vaporization stage at a pressure that is lower than the pressure of the effluent that is obtained from the last reactor, the compression of the mixture that is vaporized in a compressor, the introduction of the vaporized and compressed mixture, into at least one adiabatic reactor that contains at least one dehydration catalyst.

FIELD OF THE INVENTION

This invention relates to a process for transformation of ethanol intoethylene and in particular to a process for dehydration of ethanol.

PRIOR ART

The reaction of dehydration of ethanol into ethylene is known and hasbeen presented in detail since the end of the 19th century. It is knownthat this reaction is very endothermic, balanced, and shifted towardethylene at high temperature. The temperature drop that corresponds tothe total conversion of pure ethanol is 380° C. The reference catalystthat is often used is a monofunctional acid catalyst. The gamma-aluminais the most cited catalyst. “The Dehydration of Alcohols over Alumina.I: The Reaction Scheme,” H. Knözinger, R. Köhne, Journal of Catalysis(1966), 5, 264-270 is considered to be the basic publication on theworks on dehydration of alcohols including ethanol. The zeolites arealso used for this application, and in particular ZSM5 from the 1980s,such as, for example, in “Reactions of Ethanol over ZSM-5,” S. N.Chaudhuri & al., Journal of Molecular Catalysis 62: 289-295 (1990).

The U.S. Pat. No. 4,232,179 describes a process for dehydration ofethanol into ethylene in which the heat that is necessary to thereaction is supplied by the introduction into the reactor of a coolantmixed with the feedstock. The coolant is either water vapor that isobtained from an outside source, or an outside stream that comes fromthe process, or the recycling of a portion of the effluent of thedehydration reactor, i.e., the ethylene that is produced. Theintroduction of the mixture of the feedstock with said coolant makes itpossible to provide the heat that is necessary for keeping thetemperature of the catalytic bed at a compatible level with the desiredconversion levels. In the case where the coolant is the effluent fromthe dehydration reactor, a compressor for recycling said effluent isnecessary. However, the recycling of the ethylene that is produced bythe reaction is a drawback because the introduction of ethylene modifiesthe balance of the dehydration reaction. In addition, the ethyleneparticipates in secondary oligomerization reactions, transfer ofhydrogen and disproportionation of olefins that are reactions of anorder that is higher than 0 relative to their reagent. The increase inthe ethylene concentration from the beginning of the reaction multipliesthe formation of secondary products. The loss of ethylene is thereforemore significant, which reflects a lowering of selectivity.

The patent application WO 2007/134415 A2 describes a process fordehydration of ethanol into ethylene that is improved relative to thatof the U.S. Pat. No. 4,232,179 that makes possible a reduced investmentcost, owing to a reduced number of pieces of equipment, and a reducedoperational cost, owing to the non-use of water vapor external to theprocess. In this process, at least a portion of the effluent of thedehydration reactor (mixture of ethylene that is produced and watervapor) and the superheated water vapor obtained from the water that isproduced by the dehydration of ethanol and condensed in the reactor areused as a coolant and enter within the dehydration reactor by mixingwith ethanol. Furthermore, said patent application is silent on thepressure condition that is to be complied with between the ethanolfeedstock and the effluent for the purpose of maximizing the heatexchange.

The U.S. Pat. No. 4,396,789 also describes a process for dehydration ofethanol into ethylene in which the ethanol and the water vapor acting ascoolant are introduced into the first reactor at a temperature that isbetween 400 and 520° C. and at a high pressure of between 20 and 40 atm,in such a way that the effluent produced by the dehydration reaction isdrawn off from the last reactor at a pressure that is at least higherthan 18 atm, said reaction product, i.e., ethylene, after cooling, beingable to undergo the final cryogenic distillation stage without anintermediate compression stage. Said process is also characterized by aheat exchange between said product of the dehydration reaction and thefeedstock that is introduced into the first reactor, with said reactionproduct being used for vaporizing the feedstock that comes into thefirst reactor. The unreacted ethanol, at least a portion of the waterthat is formed during the reactions of the process, and the water thatis added for the final washing of gases are recycled to ensure thecomplete conversion of the ethanol.

One objective of the invention is to provide a process for dehydrationof ethanol into ethylene in which the feedstock is introduced into stagea) for vaporization of the feedstock at low pressure, less than thereaction pressure, such that said process does not require any coolantthat is external to the process. In particular, the feedstock isintroduced into stage a) for vaporization of the feedstock at a pressurethat is lower than the pressure of the effluent at the outlet of thelast reactor so as to maximize the heat exchange between the feedstockand the effluent that is obtained from the last reactor, i.e., toexchange the entire vaporization enthalpy of the feedstock and thecondensation enthalpy of said effluent.

Another objective of the invention is to provide a process fordehydration of ethanol into ethylene of high purity, whereby saidprocess makes it possible to increase the selectivity of ethylene with aspecific consumption per ton of ethylene that is produced that issignificantly lowered relative to the processes of the prior art.

SUMMARY AND ADVANTAGE OF THE INVENTION

The invention describes a process for dehydration of an ethanolfeedstock, which comprises a percent by mass of ethanol of between 2 and55% by weight, into ethylene comprising:

a) The vaporization of said dilute ethanol feedstock in an exchanger,owing to an exchange of heat with the effluent that is obtained from thelast reactor, with said ethanol feedstock being introduced into saidvaporization stage at a pressure that is lower than the pressure of theeffluent that is obtained from the last reactor,

b) The compression of said feedstock that is vaporized in a compressor,

c) The introduction of said vaporized and compressed feedstock, at anentrance temperature of between 350 and 550° C. and at an entrancepressure of between 0.3 and 1.8 MPa, in at least one adiabatic reactorthat contains at least one dehydration catalyst and in which thedehydration reaction takes place,

d) The separation of the effluent that is obtained from the lastadiabatic reactor of stage c) into an effluent that comprises ethyleneat a pressure that is lower than 1.6 MPa and an effluent that compriseswater,

e) The purification of at least a portion of the effluent that compriseswater that is obtained from stage d) and the separation of at least onestream of unconverted ethanol, with no recycling of said stream ofpurified water that is obtained from said stage e) being done upstreamfrom stage a).

The process uses an ethanol feedstock that is already diluted and in nocase requires recycling of the purified water that is obtained fromstage e) upstream from stage a), the water playing the role of diluentand coolant for the dehydration reaction.

This invention offers the advantage relative to the processes of theprior art for maximizing the heat exchange between the feedstock and theeffluent that is obtained from the last reactor, i.e., to exchange theentire vaporization enthalpy of the feedstock and the major portion ofthe condensation enthalpy of said effluent owing to the introduction ofthe feedstock into the vaporization stage a) at a pressure that is lowerthan that of the effluent at the outlet of the last reactor.

DESCRIPTION OF THE INVENTION

The ethanol feedstock that is treated in the process according to theinvention is optionally obtained by a process for the synthesis ofalcohol from fossil resources, such as, for example, from carbon,natural gas, and carbon waste (plastic waste, municipal waste, etc.).

Said feedstock can also advantageously come from non-fossil resources.Preferably, the ethanol feedstock that is treated in the processaccording to the invention is an ethanol feedstock that is produced froma renewable source that is obtained from biomass and is often called“bioethanol.” Said ethanol feedstock is a feedstock that is produced bybiological means, preferably by fermentation of sugar obtained from, forexample, sugar-producing crops such as sugarcane (saccharose, glucose,fructose and sucrose), beet scraps, or else amylase plants (starch), orlignocellulosic biomass or hydrolyzed cellulose (majority glucose andxylose, galactose), containing variable amounts of water.

Said feedstock is advantageously obtained by fermentation from threesources: 1) The sucrose from cane sugar or beet scraps, 2) The starchthat is present in the grains and the tubers, and 3) The cellulose andhemicellulose that are present in wood, the herbs and otherlignocellulosic biomasses, starch, cellulose and hemicellulose having tobe hydrolyzed into sugars before undergoing a fermentation stage.

At the end of fermentation, the fermented ethanol is concentrated in afirst column called a “beer column.” The ethanol feedstock that istreated in the process according to the invention advantageously comesfrom this beer column whose concentration in ethanol at the top of thecolumn is compatible with the concentration of said dilute ethanolfeedstock that is used in the process, with said feedstock comprisingbetween 2 and 55% by weight of ethanol. The use of a dilute hydratedethanol feedstock thus makes it possible not to concentrate the ethanolin a conventionally more intense manner in a second so-called“rectification” column that concentrates the ethanol toward itsazeotrope.

For a more complete description of the standard fermenting processes, itis possible to refer to the work ‘Les Biocarburants, Etat des lieux,perspectives et enjeux du développement [The Biofuels: Assessment,Perspectives and Development Issues], Daniel Ballerini, EditionsTechnip.’

Said feedstock can advantageously also be obtained by fermentation ofsynthesis gas. Said feedstock can also advantageously be obtained byhydrogenation of the corresponding acids or esters. In this case, theacetic acid or the acetic esters are advantageously hydrogenated usinghydrogen in ethanol. The acetic acid can advantageously be obtained bycarbonylation of methanol or by fermentation of the carbohydrates.

Preferably, the ethanol feedstock that is treated in the processaccording to the invention is an ethanol feedstock that is produced froma renewable source that is obtained from the biomass.

According to the invention, the ethanol feedstock that is used is anethanol feedstock that comprises a percent by mass of ethanol that isbetween 2 and 55% by weight. Said ethanol feedstock is said to bediluted. Preferably, said ethanol feedstock comprises a percent by massof ethanol of between 2 and 35% by weight. Preferably, said ethanolfeedstock is hydrated. Said ethanol feedstock also advantageouslycomprises, in addition to water, a content of alcohols other thanethanol, such as, for example, methanol, butanol and/or isopentanol thatis less than 10% by weight, and preferably less than 5% by weight, acontent of oxidized compounds other than the alcohols, such as, forexample, ethers, acids, ketones, aldehydes, and/or esters that areadvantageously less than 1% by weight, and a nitrogen and sulfurcontent, organic and mineral, advantageously less than 0.5% by weight,with the percentages by weight being expressed relative to the totalmass of the ethanol that is present in said feedstock.

The ethanol feedstock that is used according to the inventionadvantageously undergoes a pretreatment stage prior to the vaporizationstage a) of said feedstock. Said pretreatment stage makes it possible toeliminate the impurities that are contained in said feedstock in such away as to limit the deactivation of the dehydration catalyst that isplaced downstream, and in particular the metal cations, the metalanions, the compounds that contain nitrogen and the compounds thatcontain sulfur. The oxidized compounds that are present in saidfeedstock are not substantially eliminated.

Said pretreatment stage is advantageously implemented by means that areknown to one skilled in the art, such as, for example, the use of atleast one resin, by the adsorption of impurities on solids preferably ata temperature of between 20 and 200° C., by a concatenation thatcomprises a first hydrogenolysis stage that operates at a temperature ofbetween 20 and 200° C., followed by a stage for recovery on acid solidat a temperature of between 20 and 200° C. and/or by distillation. Inthe case of the use of at least one resin, said resin is preferablyacidic and is used at a high temperature of between 20 and 200° C. Saidresin can optionally be preceded by a basic resin.

In the case where the pretreatment stage is implemented by theadsorption of impurities on solids, said solids are advantageouslyselected from among the molecular sieves, activated carbon, alumina andzeolites.

Said pretreatment stage of the ethanol feedstock makes it possible toproduce a purified ethanol fraction in which the metal and organicimpurities have been eliminated, so as to obtain a purified feedstockthat responds to the level of impurities that are compatible with thedehydration catalyst.

Stage a)

According to the invention, the dehydration process comprises a stage a)for vaporization of said ethanol feedstock, optionally pretreated, in anexchanger owing to a heat exchange with the effluent that is obtainedfrom the last adiabatic reactor, with said ethanol feedstock beingintroduced into said vaporization stage at a pressure that is lower thanthe pressure of the effluent that is obtained from the last reactor.

Preferably, said ethanol feedstock being introduced into saidvaporization stage at a pressure of between 0.1 and 1.4 MPa [sic].

Preferably, at least a portion and preferably all of an unreactedethanol stream that is obtained from stage e) for purification of theeffluent that comprises water is also introduced, mixed with said dilutehydrated ethanol feedstock, optionally pretreated, in the exchanger ofthe vaporization stage a).

Preferably, said ethanol feedstock is mixed with at least a portion ofan unreacted ethanol stream that is obtained from stage e) forpurification of the effluent comprising water, after the pretreatmentstage of said ethanol feedstock.

Preferably, said ethanol feedstock, optionally mixed with at least aportion of an unreacted ethanol stream that is obtained from stage e),is introduced into said vaporization stage a) at a pressure that islower than the pressure of the effluent that is obtained from the lastreactor.

In a preferred manner, said ethanol feedstock that is optionally mixedwith at least a portion of an unreacted ethanol stream that is obtainedfrom stage e) is introduced into said vaporization stage a) at apressure that is less than the pressure of the effluent that is obtainedfrom the last reactor, with said pressure also being between 0.1 and 1.4MPa.

Thus, the pressure at which said ethanol feedstock, optionally mixed insaid vaporization stage a), is introduced is subjected to twoconditions: said pressure is to be lower than the pressure of theeffluent that is obtained from the last reactor, and, in this interval,said pressure is also advantageously to be between 0.1 and 1.4 MPa.

Actually, an essential criterion of this invention is the adjustment ofthe pressure upstream from the vaporization stage a) of said ethanolfeedstock, optionally mixed with at least a portion of an unreactedethanol stream that is obtained from stage e), in such a way as tomaximize the heat exchange between said feedstock, optionally mixed,with said unreacted ethanol stream and the effluent that is obtainedfrom the last adiabatic reactor. The introduction of said ethanolfeedstock, optionally mixed with at least a portion of said unreactedethanol stream, in the vaporization stage a) at this specific pressurelevel that is lower than the pressure of the effluent that is obtainedfrom the last reactor and preferably between 0.1 and 1.4 MPa, makes itpossible to benefit from a vaporization temperature of the possiblefeedstock mixture that is lower than the condensation temperature of theeffluent that is obtained from the last adiabatic reactor. Thus, themajor portion of the latent heat and the major portion of thecondensation enthalpy of the aqueous phase of the effluent that isobtained from the last adiabatic reactor is recovered for vaporizingsaid ethanol feedstock, optionally mixed with at least a portion of saidunreacted ethanol stream that is obtained from stage e), without anexternal heat supply. The entire vaporization enthalpy of said ethanolfeedstock, optionally mixed with at least a portion of an unreactedethanol stream that is obtained from stage e), is therefore exchangedwith the condensation enthalpy of said effluent.

The pressure of said ethanol feedstock, optionally mixed with at least aportion of said unreacted ethanol stream that is obtained from stage e),at its vaporization, is advantageously selected in such a way that thetemperature difference between the effluent that is obtained from thelast adiabatic reactor that is condensed and said feedstock mixture thatevaporates is always at least higher than 2° C., and preferably at leasthigher than 3° C.

Stage b)

According to the invention, said ethanol feedstock, optionally mixedwith at least a portion of said unreacted ethanol stream that isobtained from stage e), vaporized, undergoes compression in acompressor. The compression stage b) is advantageously implemented inany type of compressor that is known to one skilled in the art. Inparticular, the compression stage b) is advantageously implemented in acompressor of the radial compressor type with an integrated multiplieror in a compressor that comprises one or more fans with a radial wheelthat are arranged in series without intermediate cooling.

The compression stage b) of said ethanol feedstock that is optionallymixed with at least a portion of said unreacted ethanol stream,vaporized, makes it possible to prevent the supply of coolant that isexternal to the process for ensuring the vaporization of said mixture ofsaid feedstock. Thus, only the streams that are obtained from theprocess are used. The compression stage b) therefore makes it possibleto produce a heat pump that is integrated in said process, using thestreams that are obtained from the process, and not involving externalcoolant.

The combination of the specific operating conditions of stage a) andstage b) makes it possible to recover the major portion of the latentheat of said effluent and the major portion of the condensation enthalpyof the aqueous phase of the effluent that is obtained from the lastadiabatic reactor for vaporizing the entire ethanol feedstock that isoptionally mixed with at least a portion of said unreacted ethanolstream that is obtained from stage e), minimalizing the loss of latentheat and the condensation enthalpy by an external cooling and thusminimizing the supply of external heat.

The pressure of said ethanol feedstock that is optionally mixed with atleast a portion of said unreacted ethanol stream that is obtained fromstage e), vaporized at the end of the compression stage b), isadvantageously between 0.3 and 1.8 MPa. The exit pressure of saidoptional mixture of said feedstock is adequate for producing thetemperature condition that is necessary to the exchange of stage a): instage a), the vaporization temperature of said optional mixture of saidfeedstock is to be lower than the condensation temperature of theeffluent that is obtained from the last reactor.

Said ethanol feedstock that is optionally mixed with at least a portionof said unreacted ethanol stream that is obtained from stage e),vaporized and compressed, obtained from compression stage b), isoptionally heated in a gas single-phase-type exchanger, owing to a heatexchange with the effluent that is obtained from the last adiabaticreactor of stage c). In said gas single-phase-type exchanger, saidoptional mixture of said feedstock, vaporized and compressed, issuperheated, and the effluent that is obtained, in the gaseous state,from the last adiabatic reactor of stage c) is “de-superheated” withoutbeing condensed.

Said optional mixture of said feedstock is advantageously superheated toa temperature of between 250 and 375° C. and preferably between 280 and360° C. At the end of said gas single-phase-type exchanger, the effluentthat is obtained, in the gaseous state, from the last adiabatic reactorof stage c) advantageously has a temperature of between 150 and 320° C.,and preferably between 200 and 300° C.

Thus, the use of different exchangers, of the gas single-phase type andthe gas/liquid vaporizer type, and vaporization, at a pressure that islower than the pressure of the output effluent of the last reactor, ofsaid ethanol feedstock that is optionally mixed with at least a portionof said unreacted ethanol stream that is obtained from stage e), makespossible the condensation of at least 80% of the water vapors that arepresent in the effluent that is obtained from the last reactor.

Said optional mixture of feedstock—vaporized, compressed and optionallyheated in said gas single-phase-type exchanger—is next advantageouslyintroduced into a furnace in such a way as to bring it to an entrancetemperature in at least one adiabatic reactor that is compatible withthe temperature of the dehydration reaction.

Stage c)

According to the invention, said ethanol feedstock that is optionallymixed with at least a portion of said unreacted ethanol stream that isobtained from stage e), vaporized and compressed, and optionally heated,is introduced at an entrance temperature of between 350 and 550° C. andat an entrance pressure of between 0.3 and 1.8 MPa in at least oneadiabatic reactor that contains at least one fixed catalyst bed fordehydration and in which the dehydration reaction takes place.

The effluent that is obtained from the last adiabatic reactor of stagec) advantageously has, at the outlet of the last adiabatic reactor ofstage c), a temperature of between 270 and 450° C., and preferablybetween 300 and 410° C.

The effluent that is obtained from the last adiabatic reactor of stagec) advantageously has, at the outlet of the last adiabatic reactor ofstage c), a pressure of between 0.2 and 1.6 MPa.

Said pressure at the outlet of the last adiabatic reactor of stage c) isalso advantageously higher than the pressure at which said ethanolfeedstock is introduced into said vaporization stage, in such a way asto recover the major portion of the latent heat and the major portion ofthe condensation enthalpy of the aqueous phase of the effluent that isobtained from the last adiabatic reactor.

Stage c), in which the dehydration reaction takes place, isadvantageously carried out in one or two reactors.

In the case where stage c) is implemented in an adiabatic reactor, saidethanol feedstock that is optionally mixed with at least a portion ofsaid unreacted ethanol stream that is obtained from stage e), vaporizedand compressed, and optionally heated, is advantageously introduced intosaid reactor at an entrance temperature of between 350 and 550° C., andpreferably between 400 and 500° C., and at an entrance pressure ofbetween 0.3 and 1.8 MPa. The effluent that is obtained from saidadiabatic reactor advantageously has a temperature of between 270 and450° C., and preferably between 340 and 420° C., and an exit pressurethat is advantageously between 0.2 and 1.6 MPa.

In the case where stage c) is implemented in two adiabatic reactors,said ethanol feedstock that is optionally mixed with at least anunreacted ethanol stream that is obtained from stage e), vaporized andcompressed, and optionally heated, is advantageously introduced into thefirst reactor at an entrance temperature of between 350 and 550° C. andpreferably at a temperature of between 370 and 500° C., and at anentrance pressure of between 0.4 and 1.8 MPa.

The effluent that is obtained from the first adiabatic reactoradvantageously exits from said first reactor at a temperature of between270 and 450° C., and preferably between 290 and 390, and at a pressureof between 0.3 and 1.7 MPa.

Said effluent is next advantageously introduced into a furnace in such away that the entrance temperature of said effluent in the secondadiabatic reactor is between 350 and 550° C., and preferably between 400and 500° C. Said effluent has an entrance pressure in said secondreactor that is advantageously between 0.3 and 1.7 MPa.

The effluent that is obtained from the second adiabatic reactor exitsfrom said second adiabatic reactor at a temperature that isadvantageously between 270 and 450° C., and preferably between 340 and420° C. The exit pressure of said effluent that is obtained from thesecond adiabatic reactor is advantageously between 0.2 and 1.6 MPa.

The entrance temperature of the reactor(s) can advantageously begradually increased to prevent the deactivation of the dehydrationcatalyst.

The dehydration reaction that takes place in at least one adiabaticreactor of stage c) of the process according to the invention isadvantageously performed at an hourly speed by weight that is between0.1 and 20 h-1 and preferably between 0.5 and 15 h-1. The hourly speedby weight is defined as being the ratio of the mass flow rate of thepure ethanol feedstock to the mass of the catalyst.

The dehydration catalyst that is used in stage c) is a catalyst that isknown to one skilled in the art. Said catalyst is preferably anamorphous acid catalyst or a zeolitic acid catalyst.

In the case where the dehydration catalyst that is used in stage c) is azeolitic catalyst, said catalyst comprises at least one zeolite that isselected from among the zeolites that have at least pore openingscontaining 8, 10 or 12 oxygen atoms (8 MR, 10 MR or 12 MR). It isactually known to define the size of the pores of the zeolites by thenumber of oxygen atoms that form the annular cross-section of thechannels of the zeolites, called “member ring” or MR in English. In apreferred manner, said zeolitic dehydration catalyst comprises at leastone zeolite that has a structural type that is selected from among thestructural types MFI, MEL, FAU, MOR, FER, SAPO, TON, CHA, EUO and BEA.Preferably, said zeolitic dehydration catalyst comprises anMFI-structural-type zeolite and in a preferred manner a ZSM-5 zeolite.

The zeolite that is employed in the dehydration catalyst that is used instage c) of the process according to the invention can advantageously bemodified by dealuminification or desilication according to any method ofdealuminification or desilication known to one skilled in the art.

The zeolite that is employed in the dehydration catalyst that is used instage c) of the process according to the invention or the final catalystcan advantageously be modified by an agent of the type to attenuate itstotal acidity and to improve its hydrothermal resistance properties.Preferably, said zeolite or said catalyst advantageously comprisesphosphorus, preferably added in phosphate (³⁻PO₄) form followed by avapor treatment after neutralization of the excess acid by a basicprecursor, such as, for example, sodium Na or calcium Ca. In a preferredmanner, said zeolite comprises a phosphorus content of between 0.5 and4.5% by weight relative to the total mass of the catalyst.

Preferably, the dehydration catalyst that is used in stage c) of theprocess according to the invention is the catalyst that is described inthe patent applications WO/2009/098262, WO/2009/098267, WO/2009/098268or WO/2009/098269.

In the case where the dehydration catalyst that is used in stage c) isan amorphous acid catalyst, said catalyst comprises at least one porousrefractory oxide that is selected from among alumina, alumina that isactivated by a deposit of mineral acid, and silica-alumina.

Said amorphous or zeolitic dehydration catalyst that is used in stage c)of the process according to the invention can advantageously alsocomprise at least one oxide-type matrix that is also called a binder.According to the invention, matrix is defined as an amorphous or poorlycrystallized matrix. Said matrix is advantageously selected from amongthe elements of the group that is formed by clays (such as, for example,among the natural clays such as kaolin or bentonite), magnesia,aluminas, silicas, silica-aluminas, aluminates, titanium oxide, boronoxide, zirconia, aluminum phosphates, titanium phosphates, zirconiumphosphates, and carbon. Preferably, said matrix is selected from amongthe elements of the group that is formed by the aluminas, the silicas,and the clays.

Said dehydration catalyst that is used in stage c) of the processaccording to the invention is advantageously shaped in the form ofgrains of different shapes and sizes. It is advantageously used in theform of cylindrical or multilobar extrudates such as bilobar, trilobarand multilobar extrudates of straight or twisted shape, but it canoptionally be manufactured and used in the form of crushed powder,tablets, rings, balls, wheels, or spheres. Preferably, said catalyst isin the form of extrudates.

Said dehydration catalyst that is used in stage c) of the processaccording to the invention is advantageously implemented in at least onereactor, in a fixed bed, or in a moving bed.

In stage c) of the process according to the invention, the catalyststhat are used and the operating conditions are selected in such a way asto maximize the production of ethylene. The overall dehydration reactionthat is implemented in stage c) of the process according to theinvention is as follows:

C₂H₅OH→CH₂═CH₂+H₂O

The conversion of the ethanol feedstock in stage c) is advantageouslygreater than 90%, preferably 95%, and in a preferred manner greater than99%.

The conversion of the ethanol feedstock is defined, in percentage, bythe following formula: [1−(hourly output mass of ethanol/hourly inputmass of ethanol)]× x 100.

The hourly input and output mass of ethanol is measured conventionallyby gas phase chromatography of the aqueous phase.

Stage c), in which the dehydration reaction takes place, isadvantageously carried out in one or two reactors. A preferred reactoris a radial reactor that operates in upward or downward mode. Duringstage c) of the process according to the invention, the transformationof the feedstock is accompanied by the deactivation of the dehydrationcatalyst by coking and/or by adsorption of inhibiting compounds. Thedehydration catalyst is therefore to periodically undergo a regenerationstage. Preferably, the reactor is used in an alternate regenerationmode, also called a swing reactor, so as to alternate the reaction andregeneration phases of said dehydration catalyst. The objective of thisregeneration treatment is to burn the organic deposits as well as theradicals that contain nitrogen and sulfur, contained at the surface andwithin said dehydration catalyst.

The regeneration of the dehydration catalyst that is used in said stagec) is advantageously carried out by oxidation of coke and inhibitingcompounds under a stream of air or in an air/nitrogen mixture, forexample by using a recirculation of the combustion air with or withoutwater so as to dilute oxygen and to control regeneration exothermy. Inthis case, it is possible to advantageously adjust the content of oxygenat the inlet of the reactor by a supply of air. Regeneration takes placeat a pressure between atmospheric pressure (0 bar relative) and thereaction pressure. The regeneration temperature is advantageouslyselected from between 400 and 600° C.; it can advantageously vary duringregeneration. The end of the regeneration is detected when there is nolonger oxygen consumption, a sign of the total combustion of the coke.

Preferably, the effluent that is obtained from the last adiabaticreactor of stage c) is not recycled upstream from stage c), in at leastone adiabatic reactor.

The effluent that is obtained from the last adiabatic reactor of stagec) is optionally sent into a gas single-phase-type exchanger in which itis “de-superheated” without being condensed by heat exchange with thevaporized and compressed feedstock that is obtained from stage b), inwhich it is heated. Said “de-superheated” effluent is nextadvantageously sent into a second gas/liquid-type exchanger in which itis partially condensed by a heat exchange that is used to evaporate thefeedstock.

Stage d)

According to the invention, the effluent that is obtained from the lastadiabatic reactor of stage c) undergoes a separation stage d) into aneffluent that comprises ethylene at a pressure that is lower than 1.6MPa and an effluent that comprises water.

Stage d) for separation of said effluent that is obtained from the lastadiabatic reactor of stage c) can advantageously be implemented by anymethod that is known to one skilled in the art, such as, for example, bya gas/liquid separation zone, and preferably a gas/liquid separationcolumn. The effluent of the gas/liquid separation zone that comprisesethylene at a pressure that is lower than 1.6 MPa next advantageouslyundergoes compression. Said compression makes it possible to raise thepressure of said effluent to a pressure that is advantageously between 2and 4 MPa that is necessary for its final purification.

Preferably, the effluent that comprises ethylene that is separated atthe end of stage d) is not recycled in at least one adiabatic reactor ofstage c). The non-recycling of the ethylene that is separated at the endof stage d) in at least one adiabatic reactor of stage c) does not alterthe selectivity of ethylene of the process according to the invention.

At least a portion of the effluent that comprises water that is obtainedfrom stage d) is optionally recycled in separation stage d). In the casewhere at least a portion of the effluent that comprises water isrecycled, said portion of the effluent that comprises water isadvantageously cooled using cold fluid or a fluid that is obtained fromthe process and is preferably purified according to the knownpurification methods described below.

Stage e)

According to the invention, at least a portion of the effluent thatcomprises water that is obtained from separation stage d) undergoes apurification stage e), and with no recycling of said stream of purifiedwater that is obtained being done upstream from stage a).

The purification stage e) can advantageously be implemented by anypurification method that is known to one skilled in the art. By way ofexample, the purification stage e) can advantageously be implemented byuse of ion-exchange resins, molecular sieves, membranes, by addingchemical agents for adjusting the pH, such as, for example, soda oramines, and by adding chemical agents for stabilizing the products, suchas, for example, polymerization inhibitors that are selected from amongbisulfites and surfactants.

At least one purified water stream and at least one unconverted ethanolstream are next separated. The separation can advantageously beimplemented by any separation method that is known to one skilled in theart. By way of example, the separation can advantageously be implementedby distillation, the use of molecular sieves, membranes, vapor strippingor heat stripping or by absorption with solvent, such as, for example,glycol-containing solvents.

A stream that contains light gases, preferably acetaldehyde andmethanol, can advantageously also be separated.

At least a portion of said unreacted ethanol stream that is obtainedfrom the purification stage e) of the effluent that comprises water isadvantageously recycled and mixed, upstream from vaporization stage a),with the ethanol feedstock that is optionally pretreated.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 diagrammatically shows the process for dehydration of ethanol inthe case of the dehydration of a dilute ethanol feedstock that containsbetween 2 and 35% by weight of ethanol, with the remainder beingprimarily water.

The dilute ethanol feedstock is introduced into a pretreatment zone (2)via the pipe (1). The pretreated ethanol feedstock (3) is next mixed inthe pipe (5) with a portion of the unreacted ethanol stream that isobtained from the purification zone (20), via the pipe (4). Thepretreated ethanol feedstock that is mixed with a portion of theunreacted ethanol stream is introduced via the pipe (5) at a pressure ofbetween 0.1 and 1.4 MPa, in a gas/liquid exchanger E1 in which saidmixture undergoes heat exchange with the effluent that is obtained fromthe last adiabatic reactor R2 that penetrates the exchanger via the pipe(11). The latent heat and/or condensational enthalpy of the effluentthat is obtained from the last adiabatic reactor R2 is used to vaporizethe ethanol feedstock that is mixed with the unreacted ethanol stream,without an external heat supply.

The ethanol feedstock that is mixed with the unreacted ethanol stream,vaporized, is next sent via the pipe (6) into a compressor C1.

Said mixture of the feedstock, vaporized and compressed, is next sentvia the pipe (7) into a gas single-phase-type exchanger E2, in whichsaid mixture is heated owing to a heat exchange with the effluent thatis obtained from the last adiabatic reactor R2 that is introduced intoE2 via the pipe (10). In said gas single-phase-type exchanger, saidvaporized and compressed feedstock is superheated, and the effluent thatis obtained, in the gaseous state, from the last adiabatic reactor R2 is“de-superheated” without being condensed.

Said mixture of the feedstock—vaporized, compressed and heated in thegas single-phase-type exchanger E2—is next introduced into a furnace H1via the pipe (8) in such a way as to bring it to an entrance temperaturein the first adiabatic reactor R1 that is compatible with thetemperature of the dehydration reaction. The effluent that is obtainedfrom the first reactor R1 is sent into a second furnace H2 via the pipe(8 b) before being introduced into the second reactor R2 via the pipe (9b).

The effluent that is obtained from the second reactor R2 next undergoesthe two successive exchanges that are described above in the exchangersE2 and E1 via the pipes (10) and (11).

The effluent that is obtained from the exchanger E1 is sent via the pipe(12) into a gas/liquid separation column (13) where it is separated intoan effluent that comprises ethylene (14) and an effluent that compriseswater (15). A portion of the effluent that comprises water is recycledafter cooling in the column (13) via the pipe (16).

The portion of the effluent that comprises non-recycled water in thecolumn (13) is sent via the pipe (15) into a purification and separationzone (20). At least one stream of purified water (17) and at least onestream of unconverted ethanol (4) and (18) are next separated. A streamthat contains the light gases (19) is also separated.

A portion of said unreacted ethanol stream that is obtained from thestage (20) for purification of the effluent that comprises water isrecycled via the pipe (4) and is mixed upstream from the exchanger E1,with the pretreated ethanol feedstock (3).

The following examples illustrate the invention without limiting itsscope.

EXAMPLES Example 1 In Accordance with the Invention

Example 1 illustrates a process according to the invention in whichstage c) is implemented in an adiabatic reactor.

The ethanol feedstock under consideration is produced by fermentation ofwheat, without extracting gluten, by a dry-milling-type processaccording to the English term and only distilled in the first beercolumn to remain dilute.

The dilute ethanol feedstock whose composition is provided in Column 1of Table 1 is pretreated on a resin TA 801 at a temperature of 140° C.The characteristics of the pretreated ethanol feedstock are alsoprovided in Column 2 of Table 1.

Stage a)

Said dilute and pretreated ethanol feedstock is introduced, at a flowrate of 160,735 kg/h, into a mixture with 132 kg/h of unconvertedethanol that is obtained from stage e), in an exchanger El at a pressurethat is equal to 0.31 MPa.

No stream of purified water that is obtained from stage e) is recycledand mixed with said ethanol feedstock.

TABLE 1 Characteristics of the Ethanol Feedstock Before and AfterPretreatment (1) (2) Unit Ethanol Content 26.3 26.3 % by WeightAcetaldehyde 0.0169 0.0169 % by Weight Aldehydes 0.0175 0.0175 % byWeight Esters 0.003 0.003 % by Weight Higher Alcohols 0.2144 0.2144 % byWeight Methanol 0.0038 0.0038 % by Weight 1-Propanol 0.0604 0.0604 % byWeight 2-Methyl-1 Propanol 0.0551 0.0551 % by Weight 1 Butanol 0.00180.0018 % by Weight 2-Methyl-1 Butanol 0.0256 0.0256 % by Weight3-Methyl-1 Butanol 0.0715 0.0715 % by Weight Nitrogen Compounds 0.005 0% by Weight Water Content 73.2 73.2 % by Weight (1): Feedstock Ethanol(2): After Pretreatment

For the sake of simplicity, the description of the impurities in thepretreated feedstock was removed from the text below.

In stage a), the majority of the latent heat of the aqueous phase of theeffluent that is obtained from the adiabatic reactor of stage c) isrecovered for vaporizing the mixture of the feedstock and theunconverted ethanol, without an external heat supply. Thus, 90.1% of thewater that is contained in said effluent that is obtained from theadiabatic reactor of stage c) is in liquid aqueous form. Thus, 88.5 MWis exchanged between the mixture of the feedstock and two other streamsand the effluent of the reactor.

The temperature at the beginning of the vaporization of said feedstockis equal to 126° C. (at 0.27 MPa) and the final condensation temperatureof said effluent that is obtained from the adiabatic reactor is—theeffluent is—117° C. (at 0.41 MPa) [sic].

Stage b)

The mixture of the feedstock and the unconverted ethanol, vaporized,obtained from the exchanger, is next compressed in a radial compressorwith an integrated multiplier such that the pressure of said mixture ofthe feedstock and the unconverted ethanol, vaporized at the end of thecompression, is equal to 0.63 MPa.

The mixture of the feedstock and the unconverted ethanol, vaporized andcompressed, is next heated in a gas single-phase-type exchanger E2,owing to a heat exchange with the effluent that is obtained from theadiabatic reactor of stage c). In said gas single-phase-type exchanger,said mixture of the feedstock and the unconverted ethanol, vaporized andcompressed, is superheated to a temperature of 345° C., and the effluentthat is obtained, in the gaseous state, of the adiabatic reactor ofstage c) is “de-superheated” without being condensed and has atemperature of 269° C.

Stage c)

Said mixture of the feedstock and the unconverted ethanol—vaporized,compressed and heated in said gas single-phase-type exchanger—is nextintroduced into a furnace in such a way as to bring it to an entrancetemperature in said adiabatic reactor that is compatible with thetemperature of the dehydration reaction, i.e., at a temperature of 500°C.

Said mixture of the feedstock and the unconverted ethanol—vaporized,compressed and heated—is introduced into the adiabatic reactor at anentrance pressure of 0.53 MPa.

The adiabatic reactor contains a fixed bed of dehydration catalyst, withsaid catalyst comprising 80% by weight of ZSM-5 zeolite that is treatedwith H₃PO₄ in such a way that the P₂O₅ content is 3.5% by weight.

The temperature and pressure conditions of the streams entering andexiting from the adiabatic reactor of stage c) are provided in Table 2:

TABLE 2 Operating Conditions of Dehydration Stage c). Unit Entrance ExitPressure MPa 0.53 0.50 Hourly Speed by Weight h⁻¹ 7 7 ReactionTemperature ° C. 500 384

The conversion of the ethanol feedstock in stage c) is 99.4%.

Stage d)

The effluent that is obtained from the adiabatic reactor of stage c)next undergoes the two heat exchanges described above and is sent into agas/liquid separation column. An effluent that comprises ethylene at apressure that is equal to 0.39 MPa is separated as well as an effluentthat comprises water. This separation is carried out by the use of agas/liquid separation column, with recycling of the water that isproduced at the bottom of the column toward the top of the column andafter cooling and injection of neutralizing agent.

The effluent that comprises ethylene next undergoes compression forraising its pressure to 2.78 MPa before its final purification. Theseparated ethylene is not recycled in said adiabatic reactor.

Stage e)

A purified water stream and an unconverted ethanol stream as well as astream that contains light gases are next separated by conventionallow-pressure distillation from the raw water.

Stage f)

A portion of the unconverted ethanol stream is recycled upstream fromthe vaporization stage a).

Information regarding the different streams, in kg/h, is given in Tables3 and 4:

TABLES 3 and 4 Composition of the Primary Streams. Description of theStream Pretreated Stream Stream Effluent Ethanol Entering ExitingComprising Feedstock Into R1 From R1 Ethylene Stream No. Correspondingto the Figure 3 9 10 14 Total Mass Flow kg/h 160,735 160,866 160,86626,076 Rate Mass Flow Rate, kg/h by Components Ethylene 0 0 25,15425,124 Ethane 0 0 21 21 C3 0 0 88 88 C4 0 0 504 503 Oxidized Compounds 00 110 27 (Other than Ethanol) Ethanol 42,559 42,685 267 6 H₂O 118,176118,182 134,722 307 Description of the Stream Effluent That UnconvertedComprises Ethanol Purged Light Water Recycling Water Gases Stream No.Corresponding to the Figure 15 4 17 19 Total Mass kg/h 134,790 132 134,544 114 Flow Rate Mass Flow kg/h Rate, By Components Ethylene 31 0 030.9 Ethane 0 0 0 0.0 C3 0 0 0 0.1 C4 1 0 0 0.5 Oxidized Compounds 83 00 83 (Other than Ethanol) Ethanol 261 126  135 0.0 H₂O 134,415 6 134,4090.0

The compounds C3 and C4 are C3 and C4 hydrocarbon compounds.

The selectivity of the process in terms of ethylene is 97%.

It is calculated in the following way: (Ethylene that is contained inthe effluent that comprises ethylene)/(0.61 *amount of convertedethanol) where the amount of converted ethanol is the ethanol that iscontained in the pretreated ethanol feedstock that is subtracted fromthe ethanol that is contained in the streams of purged water and in theeffluent that comprises ethylene. 0.61 g is the maximum amount ofethylene that is obtained by dehydrating 1 g of pure ethanol.

Information on the energy balance of the diagram according to Example 1in accordance with the invention is given in Table 5:

TABLE 5 Energy Balance Energy Exchanged Inside Energy Provided to theSystem by an the System External Supply Amount of Amount of Amount ofHeat Heat Heat Extracted Exchanged in Exchanged in Amount of Electricityon the the First the Second Heat Required Gas/Liquid Exchanger ExchangerExchanged in for Separation (El) (E2) the Furnace Compression Column MWMW MW MW MW 88.5 10.8 15.4 8.4 13.8

The estimation of the primary energy consumption was carried out byusing the following bases:

-   -   Effectiveness of 0.8 on the furnaces    -   Effectiveness of 0.375 on the production of electricity.

The diagram according to Example 1 in accordance with the invention hasan equivalent primary energy consumption or a specific consumption of 6GJ equivalent per ton of ethylene that is produced.

Example 2 In Accordance with the Invention

Example 2 illustrates a process according to the invention in whichstage c) is implemented in two adiabatic reactors.

Stage a)

The same pretreated ethanol feedstock as the one that is used in Example1 is introduced at a flow rate of 160,735 kg/h into an exchanger E1 at apressure that is equal to 0.31 MPa, mixed with 132 kg/h of unconvertedethanol, obtained from stage e). No flow of purified water obtained fromstage e) is recycled and mixed with said feedstock.

Stage b)

The heat exchange that is described in Example 1 takes place, and themixture of the feedstock and the unconverted ethanol, vaporized, is nextcompressed in a compressor of the same type as that of Example 1 in sucha way that the pressure of said mixture of the feedstock and theunconverted ethanol, vaporized at the end of the compression, is equalto 0.69 MPa. 90.2% of the water that is contained in the effluent thatis obtained from the last reactor is in liquid aqueous form. Thus, 88.9MW is exchanged between mixing the feedstock and the two streams and theeffluent that is obtained from the last reactor.

The temperature at the beginning of the vaporization of said feedstockis equal to 126° C. (at 0.27 MPa) and the final condensation temperatureof said effluent that is obtained from the adiabatic reactor is—theeffluent is—117° C. (at 0.41 MPa) [sic].

Stage c)

The mixture of the feedstock and the unconverted ethanol, vaporized andcompressed, is next heated in a gas single-phase-type exchanger E2 owingto a heat exchange with the effluent that is obtained from the secondadiabatic reactor of stage c). In said gas single-phase-type exchanger,the mixture of the feedstock and the unconverted ethanol, vaporized andcompressed, is superheated to a temperature of 353° C., and the effluentthat is obtained, in the gasesous state, of the adiabatic reactor ofstage c) is “de-superheated” without being condensed and has atemperature of 275° C.

The mixture of the feedstock and the unconverted ethanol—vaporized,compressed and heated in said gas single-phase-type exchanger—is nextintroduced into a furnace in such a way as to bring it to an entrancetemperature in the first adiabatic reactor that is compatible with thetemperature of the dehydration reaction, i.e., to a temperature of 400°C.

The mixture of the feedstock and the unconverted ethanol—vaporized,compressed and heated—is introduced into the first adiabatic reactor atan entrance pressure of 0.62 MPa.

The effluent that is obtained from the first adiabatic reactor exitsfrom said first reactor at a temperature of 318° C. and is nextintroduced into a furnace in such a way that the entrance temperature ofsaid effluent in the second adiabatic reactor is 405° C. Said effluenthas an entrance pressure in said second reactor of 0.53 MPa.

The effluent that is obtained from the second adiabatic reactor exitsfrom said second adiabatic reactor at a temperature of 380° C. and at apressure of 0.50 MPa.

The two adiabatic reactors each contain a fixed catalyst bed fordehydration, whereby said catalyst is identical in the two reactors andidentical to the one that is used in Example 1.

The temperature and pressure conditions of the streams that enter andexit from the adiabatic reactors of stage c) are provided in Table 6:

TABLE 6 Operating Conditions of Dehydration Stage c). Reactor 1 Reactor2 Unit Entrance Exit Entrance Exit Pressure MPa 0.59 0.56 0.53 0.50Hourly Speed by h⁻¹ 14 14 Weight Reaction ° C. 400 318 405 380Temperature

The conversion of the ethanol feedstock at the end of stage c) is 99.4%.

Stage d)

The effluent that is obtained from the second adiabatic reactor of stagec) next undergoes the two heat exchanges described above and is sentinto a gas/liquid separation column. An effluent that comprises ethyleneat a pressure that is equal to 0.39 MPa is separated as well as aneffluent that comprises water. This separation is carried out by use ofa gas/liquid separation column, with recycling of the water that isproduced at the bottom of the column toward the top of the column aftercooling and injection of a neutralizing agent.

The effluent that comprises ethylene next advantageously undergoescompression for raising its pressure to 2.78 MPa before its finalpurification. The separated ethylene is not recycled in the first or thesecond adiabatic reactor.

Stage e)

A purified water stream and an unconverted ethanol stream as well as astream containing light gases are next separated by conventionallow-pressure distillation of the raw water.

Stage f)

A portion of the unconverted ethanol stream is recycled upstream fromthe vaporization stage a).

Information regarding the different streams, in kg/h, is given in Tables7 and 8:

TABLES 7 AND 8 Composition of the Primary Streams. Description of theStream Pretreated Stream Stream Stream Effluent that Ethanol EnteringEntering Exiting Comprises Feedstock into R1 into R2 from R2 EthyleneStream No. 3 9 9b 10 14 Total Mass kg/h 160,735 160,867 160,867 160,86726,076 Flow Rate Mass Flow Rate, kg/h by Components Ethylene 0 0 17,92925,155 25,123 Ethane 0 0 10 21 21 C3 0 0 11 88 88 C4 0 0 137 504 503Oxidized Compounds 0 0 599 110 27 (Other than Ethanol) Ethanol 42,55942,685 12,183 267 6 H₂O 118,176 118,182 129,998 134,727 307 Descriptionof the Stream Effluent that Unconverted Comprises Ethanol Purged LightWater Recycling Water Gases Stream No. 15 4 17 19 Total Mass kg/h134,790 132 134,544 114 Flow Rate Mass Flow Rate, kg/h by ComponentsEthylene 31 0 0 31 Ethane 0 0 0 0 C3 0 0 0 0 C4 1 0 0 1 OxidizedCompounds 83 0 0 83 (Other than Ethanol) Ethanol 261 126 135 0 H₂O134,415 6 134,409 0

The selectivity of the process in terms of ethylene is 97%. It iscalculated in the same way as for Example 1.

Information regarding the energy balance of the diagram according toExample 1 in accordance with the invention is given in Table 9:

TABLE 9 Energy Balance Energy Exchanged Inside the System EnergyProvided to the System by an External Supply Amount of Heat Amount ofHeat Amount of Amount of Heat Amount of Heat Exchanged Exchanged HeatExchanged Extracted on the in the First in the Second Exchanged in theElectricity Gas/Liquid Exchanger Exchanger in the First Second Requiredfor Separation El E2 Furnace Furnace Compression Column MW MW MW MW MWMW 88.9 9.9 4.6 8.4 9.5 14.0

The estimation of the primary energy consumption was carried out byusing the same bases as for Diagram 1.

The diagram according to Example 2 in accordance with the invention hasan equivalent primary energy consumption or specific consumption of 6 GJequivalent per ton of ethylene produced.

Example 3 For Comparison

Example 3 illustrates a process in which the dehydration reaction isimplemented in an adiabatic reactor and in which the feedstock, mixedwith an unconverted ethanol stream, is introduced at low pressure intothe vaporization stage a), and said mixture, vaporized, at the outlet ofthe exchanger does not undergo compression stage b). In this example,the separated ethylene is not recycled in said adiabatic reactor thatcontains the dehydration catalyst.

The same pretreated ethanol feedstock as that used in Example 1 isintroduced, with a flow rate of 160,618 kg/h, into an exchanger at apressure that is equal to 0.65 MPa, mixed with 132 kg/h of unconvertedethanol that is obtained from stage e). The mixture of the ethanolfeedstock with the unconverted ethanol is partially vaporized by heatexchange between said mixture and the effluent that is obtained from theadiabatic reactor. Only a portion of the condensation enthalpy of theaqueous phase of the effluent can be used to partially vaporize saidmixture of the ethanol feedstock with the unconverted ethanol. Thus,only 33.3% by weight of said mixture is vaporized, and only 12% of theaqueous effluent is condensed, which corresponds to an exchanged heatamount of 31.8 MW. So as to totally vaporize said mixture, an additionalamount of heat of 58.2 MW is to be provided by an outside heat source:said partially vaporized mixture is next totally vaporized in anevaporator-type exchanger, using the vapor as a coolant.

Said partially vaporized mixture, which is then evaporated in saidevaporator-type exchanger, is next introduced into a furnace in such away as to bring it to an entrance temperature in said adiabatic reactorthat is compatible with the temperature of the dehydration reaction,i.e., at a temperature of 500° C.

Said vaporized and heated feedstock is introduced into the adiabaticreactor at an entrance pressure of 0.53 MPa.

The adiabatic reactor contains a fixed catalyst bed for dehydration,whereby said catalyst is identical to that which is used in Example 1.

The temperature and pressure conditions in said adiabatic reactor are asfollows:

TABLE 10 Operating Conditions of Dehydration Stage c). Unit EntranceExit Pressure MPa 0.53 0.50 Hourly Speed by Weight h⁻¹ 7 7 ReactionTemperature ° C. 500 383

The conversion of the ethanol feedstock is 99.4%.

The effluent that is obtained from the adiabatic reactor of stage c)next undergoes the heat exchange described above: it is cooled up to144° C. and should be cooled in an exchanger that uses an outsiderefrigerant fluid for reaching 117° C. before being sent into agas/liquid separation column. This exchanger is a cooler that operateswith water. An amount of heat of 68 MW should thus be exchanged betweenthe effluent of the reactor and the refrigerant fluid. An effluent thatcomprises ethylene at a pressure that is equal to 0.38 MPa is separatedas well as an effluent that comprises water. This separation is carriedout by the use of a gas/liquid separation column, with recycling of thewater that is produced at the bottom of the column toward the top of thecolumn after cooling and injection of neutralizing agent.

The effluent that comprises ethylene next advantageously undergoescompression for raising its pressure to 2.78 MPa before its finalpurification. The separated ethylene is not recycled in said adiabaticreactor.

A purified water stream and an unconverted ethanol stream as well as astream that contains light gases are next separated by conventionallow-pressure distillation from the raw water.

A portion of the unconverted ethanol stream is recycled upstream fromvaporization stage a).

Information regarding the different streams, in kg/h, is provided inTables 11 and 12:

TABLES 11 AND 12 Composition of the Primary Streams Description of theStream Pretreated Stream Stream Effluent that Effluent that EthanolEntering Exiting Comprises Comprises Feedstock into R1 from R1 EthyleneWater Total Mass kg/h 160,618 160,750 160,750 26,015 134,734 Flow RateMass Flow Rate, kg/h by Components Ethylene 0 0 25,088 25,057 31 Ethane0 0 21 21 0 C3 0 0 88 88 0 C4 0 0 502 502 1 Oxidized Compounds 0 0 11028 82 (Other than Ethanol) Total Mass kg/h 160,618 160,750 160,75026,015 134,734 Flow Rate Ethanol 42,446 42,572 266 6 260 H₂O 118,172118,178 134,675 314 134,361 Description of the Stream UnconvertedEthanol Purged Light Recycling Water Gases Total Mass kg/h 132 134,489113 Flow Rate Mass Flow Rate, kg/h by Components Ethylene 0 0 31 Ethane0 0 0 C3 0 0 0 C4 0 0 1 Oxidized Compounds 0 0 82 (Other than Ethanol)Ethanol 126 134 0 H₂O 6 134,355 0

The selectivity of the process in terms of ethylene is 97%. It iscalculated in the same way as for Example 1.

Information regarding the energy balance of the diagram according toExample 3 that is not in accordance with the invention is given in Table13:

TABLE 13 Energy Balance Energy Energy Provided to the System by anExternal Supply Exchanged Amount of Inside the System Amount of Amountof Amount of Heat Extracted Amount of Heat Heat Heat on the HeatExchanged Exchanged Exchanged Extracted Gas/Liquid on the 1^(st) on theon the on the Separation Exchanger Evaporator Furnace Cooler Column MWMW MW MW MW 31.9 58.2 33.0 68.0 13.6

The estimation of the primary energy consumption was carried out byusing the same bases as for Diagram 1, by considering in addition aneffectiveness of 0.9 on the vapor production.

This Diagram 3 has an equivalent primary energy consumption, or aspecific consumption, of 15.2 GJ equivalent per ton of ethylene that isproduced. The vaporization of the feedstock that is mixed with anunconverted ethanol stream and a purified water stream, carried out inDiagram 1 of Example 1 according to this invention, at low pressure,makes it possible to reduce in a significant way the equivalent primaryenergy consumption: Diagram 1 had a primary energy consumption of 6 GJequivalent per ton of ethylene.

Example 4 For Comparison

Example 4 illustrates a process in which the dehydration reaction isimplemented in an adiabatic reactor and in which the feedstock, mixedwith an unconverted ethanol stream, is introduced into vaporizationstage a), and said mixture, vaporized, at the outlet of the exchanger,does not undergo compression stage b).

Example 4 is based on the fact that a portion of the effluent that isobtained from the adiabatic reactor, comprising ethylene and water, iscompressed and recycled at the inlet of the first reactor, this for thepurpose of recycling a portion of the coolant that is the water that isdirectly in vapor form without condensation and revaporization. Thisrecycling contains ethylene, however, and consequently, secondaryreactions of oligomerization, hydrogen transfer, and disproportionationof the olefins will take place in a larger amount on the reactor,leading to an overall loss of ethylene production on the reactor andtherefore a reduction in ethylene selectivity.

A feedstock with 35% by weight of pretreated ethanol is introduced at arate of 121,508 kg/h in an exchanger at a pressure that is equal to 0.65MPa, mixed with 128 kg/h of unconverted ethanol. In this example, therecycling of a portion of the effluent of the reactor that containswater to the input of the reactor makes it possible to ensure a dilutionrate of the ethanol at the inlet of the reactor that is comparable tothe preceding examples. Said mixing of the ethanol feedstock and theunconverted ethanol stream is partially vaporized by heat exchange withthe effluent that is obtained from the adiabatic reactor. Only a portionof the condensation enthalpy of the aqueous phase of the effluent can beused for partially vaporizing said mixture. Thus, only 38.4% by weightof said mixture is vaporized and only 22.9% by weight of the aqueouseffluent is condensed, which corresponds to an amount of exchanged heatof 34.5 MW.

Said partially vaporized mixture is next mixed with a portion of theeffluent that is obtained from the adiabatic reactor that comprisesethylene and water, previously compressed, whose flow rate is 49,765kg/h. The supply of heat that is linked to said recycled and compressedeffluent is not adequate to vaporize the entire mixture of the ethanolfeedstock mixed with the unconverted ethanol: 68% by weight of saidmixture is vaporized. So as to totally vaporize said mixture, it isnecessary to provide an additional 31.5 MW by an external heat source:said partially vaporized mixture is next vaporized totally in anevaporator-type exchanger that uses vapor as a coolant.

Said mixture that is vaporized and heated in said evaporator-typeexchanger is next introduced into a furnace in such a way as to bring itto an entrance temperature in said adiabatic reactor that is compatiblewith the temperature of the dehydration reaction, i.e., to a temperatureof 490° C.

Said vaporized and heated feedstock is introduced into the adiabaticreactor at an entrance pressure of 0.53 MPa.

The adiabatic reactor contains a fixed catalyst bed for dehydration,whereby said catalyst is identical to the one that is used in Example 1.

The temperature and pressure conditions in said adiabatic reactor are asfollows:

TABLE 14 Operating Conditions. Unit Entrance Exit Pressure MPa 0.53 0.50Hourly Speed by Weight h⁻¹ 7 7 Reaction Temperature ° C. 490 393

The conversion of the ethanol feedstock is 99.5%.

The effluent that is obtained from the adiabatic reactor next undergoesthe heat exchange that is described above, and is cooled to 117° C. byan outside source before being sent into a gas/liquid separation column.This exchanger can be a cooler that operates with water. An amount ofheat of 37.3 MW should thus be exchanged between the effluent of thereactor and the refrigerant fluid. An effluent that comprises ethyleneat a pressure that is equal to 0.43 MPa is separated as well as aneffluent that comprises water. This separation is carried out by the useof a gas/liquid separation column, with recycling of the water that isproduced at the bottom of the column to the top of the column aftercooling and injection of neutralizing agent.

The effluent that comprises ethylene next advantageously undergoescompression for raising its pressure to 2.78 MPa before its finalpurification. The separated ethylene is not recycled in said adiabaticreactor.

A purified water stream and an unconverted ethanol stream as well as astream that contains light gases are next separated by conventionallow-pressure distillation of the raw water.

An unconverted ethanol stream is recycled upstream from the vaporizationstage a).

Information regarding the different streams, in kg/h, is given in Table15:

TABLE 15 Composition of the Primary Streams. Description of the StreamsExchanger Recycling of Stream Stream Pretreated Entrance the EffluentEntering Exiting Ethanol Recombined Obtained from the from the FeedstockLoad the Reactor Reactor Reactor Total Mass kg/h 121,508 121,636 49,765171,401 171,400 Flow Rate Mass Flow Rate, kg/h by Components Ethylene 00 9,433 9,433 32,488 Ethane 0 0 8 8 28 C3 0 0 264 264 909 C4 0 0 753 7532,595 Oxidized Compounds 0 0 28 28 98 (Other than Ethanol) Ethanol42,451 42,573 91 42,664 313 H₂O 79,057 79,063 39,111 118,174 134,707 C4+0 0 77 77 264 Description of the Streams Effluent Effluent EffluentGoing to that that Recycling of the Comprises Comprises UnconvertedExchanger Ethylene Water Ethanol otal Mass kg/h 121,636 26,012 95,624128 Flow Rate Mass Flow Rate, kg/h by Components Ethylene 23,056 23,02530 0 Ethane 20 20 0 0 C3 645 644 1 0 C4 1,842 1,840 2 0 OxidizedCompounds 69 23 47 0 (Other than Ethanol) Ethanol 222 6 216 122 H₂O95,596 269 95,327 6 C4+ 187 185 2 0 Description of the Streams PurgedWater Light Gases Total Mass kg/h 95,414 82 Flow Rate Mass Flow Rate, byComponents kg/h Ethylene 0 30 Ethane 0 0 C3 0 1 C4 0 2 OxidizedCompounds 0 47 (Other than Ethanol) Ethanol 95 0 H₂O 95,321 0 C4+ 0 2

The selectivity of the process in terms of ethylene is 89%. It iscalculated in the same way as for Example 1. The loss of selectivitythat is linked to the recycling of the effluent that is obtained fromthe adiabatic reactor comprising ethylene and water is noted, with thepreceding diagrams not implementing the recycling of said effluent thatcomprises ethylene, making it possible to obtain selectivity in terms ofethylene of 97%.

Information regarding the energy balance of the diagram according toExample 4 that is not in accordance with the invention is given in Table16:

TABLE 16 Energy Balance Energy Provided to the System by an ExternalSupply Energy Exchanged Amount of Amount Amount of Heat Inside theSystem Heat Amount of Electricity of Heat Extracted on the Amount ofHeat Exchanged on Heat Required Extracted Gas/Liquid Exchanged on theExchanged on for the on the Separation the First Exchanger Evaporatorthe Furnace Compressor Cooler Column MW MW MW MW MW MW 34.5 31.5 34.80.37 37.3 13.6 Internal Source External External External ExternalExternal Source Source Source Source Source

The estimation of the primary energy consumption was carried out byusing the same bases as for Diagram 1, by considering in addition aneffectiveness of 0.9 on the vapor production.

This Diagram 4 has an equivalent primary energy consumption or aspecific consumption of 13.5 GJ equivalent per ton of ethylene that isproduced. The vaporization of the feedstock mixed with an unconvertedethanol stream, carried out in Diagram 1 of Example 1 according to thisinvention, at low pressure, makes it possible to reduce in a significantway the equivalent primary energy consumption: Diagram 1 had a primaryenergy consumption of 6 GJ equivalent per ton of ethylene.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

The entire disclosures of all applications, patents and publications,cited herein and of corresponding French application No. 11/03075, filedOct. 7, 2011, are incorporated by reference herein.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. Process for dehydration of an ethanol feedstock, which comprises apercent by mass of ethanol of between 2 and 55% by weight, into ethylenecomprising: a) The vaporization of said dilute ethanol feedstock in anexchanger, owing to an exchange of heat with the effluent that isobtained from the last reactor, said ethanol feedstock being introducedinto said vaporization stage at a pressure that is lower than thepressure of the effluent that is obtained from the last reactor, b) Thecompression of said feedstock that is vaporized in a compressor, c) Theintroduction of said vaporized and compressed feedstock, at an entrancetemperature of between 350 and 550° C. and at an entrance pressure ofbetween 0.3 and 1.8 MPa, in at least one adiabatic reactor that containsat least one dehydration catalyst and in which the dehydration reactiontakes place, d) The separation of the effluent that is obtained from thelast adiabatic reactor of stage c) into an effluent that comprisesethylene at a pressure that is lower than 1.6 MPa and an effluent thatcomprises water, e) The purification of at least a portion of theeffluent that comprises water that is obtained from stage d) and theseparation of at least one stream of unconverted ethanol, with norecycling of said stream of purified water that is obtained from saidstage e) being done upstream from stage a).
 2. Process according toclaim 1, in which said ethanol feedstock is an ethanol feedstock that isproduced from a renewable source that is obtained from biomass or wastecontaining hydrocarbon or carbohydrate components.
 3. Process accordingto claim 1, in which said ethanol feedstock comprises a percent by massof ethanol of between 2 and 35% by weight.
 4. Process according to claim1, in which at least one unreacted ethanol stream that is obtained fromstage e) for purification of the effluent that comprises water is alsointroduced into the exchanger of vaporization stage a).
 5. Processaccording to claim 1, in which said ethanol feedstock is also introducedinto said stage a) for vaporization at a pressure of between 0.1 and1.4.
 6. Process according to claim 1, in which the pressure of theethanol feedstock, vaporized and compressed at the end of compressionstage b), is between 0.3 and 1.8 MPa.
 7. Process according to claim 1,in which said ethanol feedstock, vaporized and compressed and obtainedfrom compression stage b), is heated in a gas single-phase-typeexchanger, owing to a heat exchange with the effluent that is obtainedfrom the last adiabatic reactor of stage c).
 8. Process according toclaim 1, in which the effluent that is obtained from the last adiabaticreactor of stage c) has a temperature of between 270 and 450° C. at theoutlet of the last adiabatic reactor of stage c).
 9. Process accordingto claim 1, in which the effluent that is obtained from the lastadiabatic reactor of stage c) has a pressure of between 0.2 and 1.6 MPaat the outlet of the last adiabatic reactor of stage c).
 10. Processaccording to claim 1, in which in stage c), the dehydration reaction iscarried out in one or two reactors.
 11. Process according to claim 1, inwhich in stage c), the dehydration reaction is carried out in fixed-bedreactors characterized by a downward or upward flow.
 12. Processaccording to claim 1, in which in stage c), the dehydration reactiontakes place, is carried out in at least one fixed-bed or moving-bedreactor that is characterized by a radial flow.
 13. Process according toclaim 1, in which said dehydration catalyst that is used in stage c) isan amorphous acid catalyst or a zeolitic acid catalyst.
 14. Processaccording to claim 1, in which at least a portion of said unreactedethanol stream that is obtained from stage e) is recycled and mixed withthe ethanol feedstock that is upstream from stage a) for vaporization ofsaid feedstock.